Method of controlling growth of brine wells

ABSTRACT

A method of solution mining of soluble salt and mineral deposits with an aqueous solvent to control the shape and dimension of a subterranean cavity, by maintaining an immiscible, liquid petroleum pad (oil pad) between the interface of the soluble deposit and the aqueous solvent used to dissolve the deposit. Since only the deposit below the petroleum oil pad is exposed to the action of the solvent, the shape of the cavity can be readily controlled by periodically raising the level of the oil pad in the well. The present process provides an improved method for solution mining whereby the depth and width of the well cavity is optimized for maximum life and minimum pumping costs.

United States Patent [72] Inventors Gordon Blair French Mendham, N.J.; Michael Slezak, Tully, N.Y. [21] Appl. No. 8,652 [22] Filed Feb. 4, 1970 [45] Patented Jan. 4, 1972 [73] Assignee Allied Chemical Corporation New York, N .Y.

[54] METHOT) OF CONTROLLING GROWTH OF BRINE l,923,896 8/1933 Trump Primary ExaminerErnest R. Purser AttorneysGerard P. Rooney and Albert L. Gazzola ABSTRACT: A method of solution mining of soluble salt and mineral deposits with an aqueous solvent to control the shape and dimension of a subterranean cavity, by maintaining an immiscible, liquid petroleum pad (oil pad) between the interface of the soluble deposit and the aqueous solvent used to dissolve the deposit. Since only the deposit below the petroleum oil pad is exposed to the action of the solvent, the shape of the cavity can be readily controlled by periodically raising the level of the oil pad in the well. The present process provides an improved method for solution mining whereby the depth and width of the well cavity is optimized for maximum life and minimum pumping costs.

WM, f lm FATE-"IEO JR! 4 B72 SHEET 2 BF 2 INVENTOR. GORDON B. FRENCH MICHAEL SLEZAK FIG. 3

AGEN

METHOD OF CONTROLLING GROWTH OF BRINE WELLS BACKGROUND OF THE INVENTION The mining of soluble salt and mineral deposits from underground strata, particularly the solution mining of salt, is well known and developed art. A number of salt fields have been mined by drilling into the salt deposits, pumping a solvent, such as water, into the hole to dissolve the salt, and pumping the resulting brine out of the hole to recover the salt.

It is well known in the art that the solution mining of salt from a single cavity has particular difficulty due primarily to the tendency of the fresh water to rise while the more dense brine settles to the bottom of the cavity. Because of this phenomenon, most of the dissolving action takes place in the upper sections of the cavity and as a result the cavity develops primarily upwardly and only secondarily laterally until it reaches overlying insoluble rock, at which point the cavity spreads only laterally on the top in a fan shape, so that a vertical cross section of the cavity resembles a moming glory." Such a cavity fails to provide the necessary structural support for the overlying loose insoluble rock (limestone, anhydrite, dolomite, etc.) and long before the salt has been mined, the insoluble rock, above the salt deposit, collapses into the brine, thus forming a semiporous insoluble blanket at the narrow bottom of the cavity and to some degree along the walls. As the moming glory" shaped cavity develops, the petals develop further and further outwardly and more of the unsupported roof rock falls. As a consequence, the falling rock and accumulation on the floor of the well, the pipe and casings often are broken and impede circulation of the solvent to the point that ultimately the well must be abandoned, leaving a very large percentage of the salt bed unmined.

Attempts to mine a salt area effectively by drilling and mining a series of such cavities in close proximity to each other, i.e., approximately 300 feet, have been generally unsuccessful not only because of the blockage due to the collapse of roof rock, but also because of the generally unstable rock stress conditions adjacent a developed cavity.

A recent development in solution mining known as the gallery method, involves two or more wells driven into the salt stratum. The stratum is split, usually hydraulically, to open a connecting passageway from one well to the other and a solvent is circulated through the underground system. This method requires an extended period of solution operation to produce an interconnecting gallery. Fractures produced by this method are not necessarily limited to the base of the salt horizon. If a fracture occurs near the top of the salt horizon, it is not long before the roof of the cavity weakens and collapses due to the removal of the supporting salt structure of the cavities, and sometimes even subsidence of the earth above the wells occurs.

One of the most effective methods, before the present invention for retarding the morning glory" effect is disclosed in US. Pat. No. 1,923,896. The invention in this patent relates to the maintenance of a layer of air or gas between the dissolving liquid and the roof of the cavity, which prevents the liquid from coming in contact with the overhead salt deposits. This method, although partially efl'ective in delaying the premature collapse of the mine cavity, requires a continuous supply of air or other gas supply pumped into the cavity, because of the dissolution of the gas in the dissolving liquid. Furthermore, after several years of operation, the mine must be prematurely abandoned because of collapsing walls within the cavity.

Because this morning glory effect has plagued the recovery of brine from salt deposits for a considerably long time, countless proposals have been made designed to increase the production of salt from a mine, some of which are described above, all of which contribute to the solution of the problem but leave much to be desired.

SUMMARY OF THE INVENTION According to the present invention, a substantially cylindrical cavity is formed in the solution mining of soluble deposits such as salt, by the use of an oil pad to separate the solvent from the overhead soluble in the developing cavity. A well is first drilled into the salt deposit and fitted to a depth of several hundred feet into the salt deposit with a cemented casing. The lower end of the casing establishes the eventual roof of the final cavity. A salt roof of sufficient thickness must be left to ensure against subsidence, after the well is abandoned, as recognized in the art. Within the cemented casing, and concentric to it, are hung two strings of pipe, the inner one extending below the outer pipe to a point near the bottom of the drilled well.

In operation, a solvent, such as water, is pumped in the annular space between the two strings of pipe, delivered at a point usually about 75 feet above the bottom of the drilled well. A water-immiscible petroleum liquid fraction of lesser density than water, is pumped through the annular space between the cemented casing and the outer string of pipe to form an oil pad on the surface of the water. The petroleum liquid may be used to fill the cavity to any depth desired, to form an oil pad, thus protecting the exposed salt all the way down to the interface and exposing the salt below the interface to the action of the water.

The level of the oil pad is controlled by surface pressure readings with addition and removal of petroleum liquid, as required, to maintain a fixed pressure. Oil pad pressures are produced at each level based upon the specific gravity of the petroleum liquid used, the specific gravity of the brine and the friction losses of the operating system. This pressure is recorded and selected levels are maintained for the duration of the development of a cavity section, by removing or adding oil as needed.

Once formed, the initial cavity of limited height and width serves as a receptacle to receive rock insolubles which are released from the stratum and fall to the floor, when the salt is dissolved. The desired initial radius of the cavity which may be in the order of 150 feet, is verified by recording the specific gravity and quantity of brine and calculating the salt removed, or through the use of sonar logging instruments. The petroleum liquid oil pad is then raised to a new level, about 50 feet up the well, above the previous level by withdrawing sufficient petroleum from the cavity. The withdrawal of petroleum liquid will result in a pressure change of the oil pad which does not affect the brining process. In actual practice it has been found that operating oil pressures in the order of about 50 to 1,000, and preferably to 750 p.s.i. (surface) may be used. For instance, a pressure of about 600 p.s.i. (surface) has been found suitable to maintain an oil pad in a well of about 3,900 feet deep, with pressure variances of about 7.5 p.s.i. suitable to move the oil pad in 50-foot increments in a vertical direction.

With the oil pad removed from the roof area over the diameter of the well, greatly increased production of saturated brine is obtained, since brine density is a function of surface area of salt exposed to dissolution. The oil pad reforms between the interface of the new roof of the cavity and the aqueous solvent, as the salt is dissolved. The volume of the oil pad is adjusted as needed to maintain the pressure recorded at the new level. Controlled slices of salt are removed in a regu lated manner for the remaining life of the well, by periodically raising the level of the oil pad.

ln dissolving sufficient salt to produce the initial cavity of desired width and height, an adequate flow of aqueous solvent through the well may be maintained to provide an uphole velocity high enough to hold insolubles, such as gypsum sands, in suspension. These insolubles may be dropped out by running the weak brine through a fully developed well, or by installing a sand collector, should the brine be of sufficient saturation to use. When using the former method, a single pump can be used to pump fresh solvent into the developing well, force the weak brine and sand out, reinject this brine down a fully developed well, and force the saturated clean brine out. However, if rapid development is not essential, a saturated salt solution may be produced from a developing well simply by operating at a low rate of flow, thus eliminating the need for reworking weak brine or disposal thereof. The specific gravity of brine product may be about 1.2 or slightly higher.

The water-immiscible liquid petroleum used in solution mining, according to this invention, may be any water-immiscible, liquid petroleum fraction which may be available at the mining site, having a specific gravity in the well cavity less than that of the aqueous solvent, crude oil being usually readily available. The specific gravity of the petroleum liquid is preferred at less than I, but may be higher or lower, and preferably in the range of about 0.7-1.0. It is only essential, the the petroleum liquid have a specific gravity less than the aqueous solvent in the cavity, so that the oil will float on the surface of the solvent and prevent the dissolution of solubles from the roof of the developing cavity. The petroleum, although expensive, is recoverable and reusable for an unlimited number of times. Crude oil is preferred because of its availability and low cost.

The present method makes possible the formation of an essentially cylindrical-shaped cavity, thus optimizing the depth and width of the well for maximum life and minimum pumping costs. The water string need not be raised or lowered. It remains in a fixed position using the present process, thus eliminating the need and expense of setting a rig over the well each time the oil level is raised.

Soluble deposits as used herein the the generic sense include not only sodium chloride, but other soluble salts such as for example, sodium nitrate, potassium chloride, epsom salt, kiersite, calcium chloride, trons, and the like, and includes the mining of sulfur by other solution mining methods in the removal of minable ores by solution or solvent mining.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings, which illustrate by way of example an embodiment in the mining solubles according to the present invention:

FIG. 1 represents a vertical section of a well drilled to a depth of about 4,000 feet, in its initial stage of operation.

FIG. 2 represents a verticalsection of the well of FIG. 1 after solution mining has been developed to a height of about 50-75 feet.

FIG. 3 represents another vertical section showing an almost complete cylindrical cavity formed in the salt deposit by the present process, nearing the terminal state of mining. The cavity may be as much as 500 feet in diameter and 3,000 feet deep.

Similar numbers relate to similar parts throughout the several views.

In FIG. 1, the well 9 is cased with 20-inch casing l cemented through the gravel and rock 2 to a point 3 about 500-600 feet below the top of the salt deposit 4. A water feedpipe 5 of 13%- inch diameter size extends down through the outer cemented casing l and through the well to a point 6 about 75-100 feet from the bottom of the well. A string of brine return pipe 7 of 8%-inch diameter size is positioned within the larger pipe 5, down the well shaft to a point 8 about 50 feet from the bottom of the well 9, the drilled well 9 being about 4,000 feet deep.

In operation water 16 is introduced through the inlet 10 under pressure of about 200 pounds per square inch down the water feedpipe 5 into the developing cavity 11, dissolving salt as it flows through the cavity and the resulting brine 12 is forced up the brine return pipe 7, exiting from the well through brine outlet pipe 13 to a storage tank (not shown).

Not shown in the diagram are conventional pumps used to force the water down and out of the mine shaft and valves and equipment associated therewith.

An oil supply 14, such as a crude oil having a density of less than 1.0 is introduced into an oil inlet 15 and oil is delivered between casing l and pipe 5 to a point above the bottom 6 of water pipe 5, thus preventing the water from attacking the salt in the well shaft. The salt is then mined and an initial cavity I7 forms as shown in FIG. 2. It is mined at a low rate of flow initially, so that usable brine can be produced from the developing well, thus eliminating the need for weak brine disposal or the reworking of weak brine.

Reference is made to FIG. 2 which illustrates the first cavity 17 formed, having a dimension of about 300 feet in diameter by about feet high, and shows the insolubles 18 on the floor of the cavity 17. The oil 14 develops an oil pad 19 which separates the aqueous layer from the roof of the cavity. As the salt is dissolved a cavity is developed as shown in FIG. 2. The oil finds its own level across the roof of the cavity to form a pad 19 separating the exposed salt along the roof from the water entering the cavity, thus forcing dissolution of the salt in an outwardly direction rather than an upwardly direction. Once formed, the initial cavity of desired height and width serves as a receptacle to receive much of the insolubles that are produced by the well. The pressure on the oil is then reduced by raising the oil level to a new height of about 50 feet above the previous level. In actual practice, operating oil pressures are in the order of 600 p.s.i. (surface) for a well of this size (about 3,900-4,000 feet deep) with about 7.5 p.s.i. variance found suitable to move the oil 50 feet in height. When the oil is removed from the roof over the diameter of the well, increased production of saturated brine is achieved, since brine density is a function of surface area of salt exposed to dissolution. In the mining of vertically elongated salt deposits production of upwards to 1,000 gallons per minute per well of saturated brine are ultimately obtained.

FIG. 3 shows the vertically elongated well during the last stages of solution mining of the well. The oil pad 19 again protects the roof of the cavity from attack by the solvent, while the sides of the cavity remain exposed to dissolution. Actually, this cylindrical cavity may be several hundred feet wide and about several thousand feet high. A belt of salt 20 of several hundred feet thick is left unmined to prevent cave-in of the overhead gravel and rock 2. The thickness of this unmined salt belt depends upon the height of the cavity below. A cavity of this size, for instance, would require a salt safety belt of 500600 feet thick.

We claim:

1. A process for recovering a soluble salt from an underground deposit which comprises a. drilling and encasing a borehole from the ground surface to a depth of about 500-600 feet below the top of the uppermost salt layer,

b. continuing drilling the borehole to a depth of up to about 4,000 feet,

c. inserting an aqueous solvent feedpipe within the casing to a depth of from 75 to 100 feet from the bottom of the borehole,

d. inserting a brine outlet pipe within the aqueous solvent feedpipe and extending downwardly to about 50 feet from the bottom of the borehole,

e. maintaining a water-immiscible petroleum liquid between the aqueous solvent feedpipe and the casing under pres sure sutficient to maintain a petroleum liquid layer just above the aqueous solution in the borehole while developing a well cavity of about 300 feet in diameter by continually feeding an aqueous solvent into the cavity and removing brine, and

f. mining the salt from the bottom of the well upwardly in increments of about 50 feet by removal of the petroleum liquid to reduce the pressure and raise the petroleum liquid level about 50 feet while maintaining the aqueous solvent feedpipe and brine outlet pipe in their original positions.

2. A process according to claim 1 wherein the aqueous solvent feed is maintained at a low rate in the initial stages of development of the well cavity and is increased as the capacity about 50 to 1,000 p.s.i. of the well cavity increases. 5. The process of claim 1 wherein the petroleum liquid is a 3. A process according to claim 1 wherein the salt is sodium Crude ilchloride. 6. The process of claim 1 wherein the oil pad comprises a 4. The process of claim 1 wherein the pad of liquid petrole- 5 Peuoleum liquid haVinB a density 0f um is maintained in the cavity under a surface pressure of 

2. A process according to claim 1 wherein the aqueous solvent feed is maintained at a low rate in the initial stages of development of the well cavity and is increased as the capacity of the well cavity increases.
 3. A process according to claim 1 wherein the salt is sodium chloride.
 4. The process of claim 1 wherein the pad of liquid petroleum is maintained in the cavity under a surface pressure of about 50 to 1,000 p.s.i.
 5. The process of claim 1 wherein the petroleum liquid is a crude oil.
 6. The process of claim 1 wherein the oil pad comprises a petroleum liquid having a density of 0.7-1.0. 